1. Field of the Invention
The present invention relates to an optical element for recording, erasing and reproducing informations to an information recording medium such as an optical disc, and to an apparatus with the optical element.
2. Description of the Prior Art
An example of a conventional optical element, more specifically a conventional waveguide type pickup used in an optical disc is shown in FIG. 20. This example is shown in Japanese Patent Unexamined Publication No. 61-296540. In FIG. 20, there are shown a semiconductor laser 10, a substrate 20, a waveguide layer 40, a dielectric layer 30 between the substrate 20 and the waveguide layer 40, a disc 50, signal pits 52, a laser beam 60, optical detectors 70, a beam splitter 80, a light focusing or converging grating coupler 90 used as an optical path changing means.
The waveguide type optical pickup is produced as follows. The dielectric layer 30 is formed on the substrate 20 by means of oxidation, vapor deposition or the like. Further, the waveguide layer 40 is formed by vapor deposition or sputtering. The beam splitter 80 the light converging grating coupler 90 and the like are formed on the other dielectric layer formed on the waveguide layer 40 by a photolithographic process, an electronic beam exposure process or a plasma etching process.
The operation of the conventional waveguide type optical pickup thus structured will now be explained. The laser beam 60 emitted from the semiconductor laser 10 is propagated through the waveguide layer 40 into the beam splitter 80. A zero-order beam that has not been diffracted is focused or converged onto the signal pit 52 on the disc 50 by the light converging grating coupler 90. A reflective beam from the disc 50 is again introduced into the waveguide layer 40 by the light converging grating coupler 90 to become a returning waveguided light advancing in a direction opposite to the emission beam and is introduced into the beam splitter 80 which is composed of two light-focusing or converging gratings to divide the returning waveguided beam into two halves to be introduced into the optical detectors 70. The detectors 70 are positioned on both sides of the light source 10. One each side, the detector 70 is divided into two parts and is used for picking up the focus error signal produced when the laser beam 60 is projected from the semiconductor laser 10 to the disc 50 and the tracking error signal and for reproducing the recording information of the signal pit 52. FIG. 20 shows a case where the tracking error is detected by a Foucault method or a push-pull method. The detailed explanation therefor is omitted because these methods are known and not essential for explanation of the present invention.
An access time of an information recording medium such as an optical disc that is a large capacity memory means is generally determined by how fast the optical pickup can be moved by a linear motor. The speed of the pickup depends upon the weight of the pickup. For this reason, in order to reduce the access time lag, there have been attempts to make the pickup light in weight and small in size. Thus, the studies and development of an integrated waveguide type optical pickup have been continued.
On the other hand, in an optical pickup, when a reflective light from a disc surface is returned back to the light source, the disc surface serves as an external resonator to make the oscillating condition of the light source unstable, resulting in reduction of a S/N (Signal-to-Noise) ratio of the reproduced signal. For this reason, in a generally used bulk type optical pickup, the returning light to the light source is avoided by, for example, combination of a 1/4 wavelength plate and a polarizing beam splitter.
However, any waveguide type optical pickup that has been proposed suffers from a serious problem. Namely, the primary or first order diffracted light of the beam splitter 80 in the returning waveguide light can be converged into the optical detector 70, but the zero order diffracted light is returned back to the semiconductor laser 10. Thus, the conventional waveguide type optical pickup suffers from such a problem that S/N ratio of the reproduced signal is lowered.